The only reason I do mognet is for the reading of certain letters most of them are qutie funny to read. Also if you are like my missus who thinks Moogles are just so cute you can do a sidequest at the end of the game that allows you to enter Mognet Central which is to be honest very cute! Also later on in the game you get Kupo Nuts from delivering letters and if you then go back to Gizamalukes Grotto and give the Kupo Nut to the Moogle couple you met there the first time you were there they wil; give you an item. It is optional but entirely up to you.